Glass ribbon width control method in float process

ABSTRACT

In a float process of flat glass production, the width of a glass ribbon formed by discharging a molten glass onto a molten metal bath is controlled in response to the temperature of a molten glass to be supplied to the molten metal bath by regulating a tweel to manipulate the rate of supplying a molten glass to the bath. Further to minimize a fluctuation of the glass ribbon width, either of two different feedback control modes, an integral control mode and a proportional-plus-integral control mode, is employed depending on whether or not an error between a measured width and a desired value is within a predetermined range of the error. Preferably the control is achieved in accordance with a sampled-data control system.

BACKGROUND OF THE INVENTION

The present invention relates to a method for controlling the width of a glass ribbon manufactured through a float process and more particularly to a control method in which the rate of supplying a molten glass onto a molten metal in a bath is manipulated by regulating the height or opening of a tweel so as to reduce an error between a measured glass ribbon width and a predetermined standard width.

In a float process of flat glass production, the rate of supplying a molten glass fluctuates depending on operational conditions of a melting furnace to make molten glass, for example, a rate of charging the furnace with raw materials and regulating condition of a heavy-oil burner of the furnace. The inventor of the present invention has found that when the temperature of a molten glass increases bringing about a lowering of the viscosity of the molten glass and hence an increase of the rate of molten glass supply, the thickness of the glass ribbon is scarcely changed but remains within a dimensional range required by specifications of production while the width of the glass ribbon grows larger. That is, a fluctuation of the rate of molten glass supply has substantially no effect on the thickness of the glass ribbon but has a material effect only on the width of the glass ribbon.

Because a molten glass in general has a high viscosity, a glass ribbon on a molten metal does not exhibit a stable width when hot just after discharged onto the molten metal bath. Accordingly a detecting device to find a glass ribbon width by detecting positions of both side edges of the glass ribbon must be placed sufficiently distant from the tweel to the downstream and therefore there is a substantial time delay between a change of the tweel height and a responsive change of the glass ribbon width.

In such a process the value of the glass ribbon width tends to oscilate in a large amplitude alternately going over and under a standard value and therefore it has been very difficult to maintain satisfactorily the glass ribbon width uniform.

A fluctuation of the glass ribbon width causes an increase of area of the both side portions of the glass ribbon which must be trimmed off for final products, resulting in a decreased yield of flat glass products. In some cases the glass ribbon on the molten metal bath is held at the both side portions thereof by supporting rolls or the like and this causes a further increase of the trimmed off portions.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method for controlling the width of a glass ribbon produced through a float process which method is so devised to minimize a fluctuation of the glass ribbon width and enhance the yield of flat glass products.

In a float process of flat glass production, a molten glass is poured through a movable tweel or sluice gate onto a molten metal in a bath to form a floating glass ribbon which advances along the molten metal bath and is gradually cooled until it becomes dimensionally stable.

To minimize a fluctuation of the glass ribbon width in such a float process, the present invention proposes the control method which comprises the steps of;

detecting the width of the glass ribbon at a location remote from the tweel,

comparing the detected width with a predetermined standard and generating an error signal representing the error therebetween,

discriminating between a first case where said error signal is within a predetermined range of the error and a second case where said error signal is outside said range,

generating a first feedback control signal from said error signal in accordance with an integral control mode in the first case and a second feedback back control signal obtained from said error signal in accordance with a proportional-plus-integral control mode in the second case,

detecting the temperature of a molten glass to be supplied to the bath and generating a temperature signal representing the detected temperature,

generating a first feedforward control signal from said temperature signal and

regulating the tweel to manipulate the rate of supplying a molten glass to the bath under command of said first feedback control signal in the first case and said second feedback control signal in the second case in such a manner as to make said error disappear and additionally under command of said first feedforward control signal in such a manner as to compensate the influence of the temperature change of the molten glass to be supplied to the bath.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing a molten metal bath chamber.

FIG. 2 is a longitudinal section of a portion of FIG. 1.

FIG. 3 is a block diagram according to the present invention.

FIG. 4 is a schematic diagram showing an example of the relationship between the error signal and the control signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1-3, a canal 1 connects a melting furnace or tank (not shown) making a molten glass with a bath chamber 2 having a molten metal bath 3 therein. And there is provided in the canal 1 a tweel or sluice gate 4 which is movable up and down by the aid of a driving device 5 and also there is provided at the end of the canal 1 a lip tile 6. A radiation thermometer 20 is placed near the tweel 4 (preferably just before or just behind the tweel 4) to detect the temperature of the molten glass at that portion. At a location sufficiently distant from the lip tile 6 to the downstream along the molten metal bath, there are placed a pair of photo-electric position detectors 7,7' such as video-analizers confronting each other with the glass ribbon between for detecting positions of both side edges and producing position signals.

Reference numeral 9 is a width signal generator circuit which carries out arithmetic operation receiving as its input the position signals from the video-analizers and generates a width signal representing the width of the glass ribbon 8. Reference numeral 10 is a standard width signal supplier circuit which generates a predetermined standard width signal and reference numeral 11 is an error signal generator circuit which compares the width signal with the standard width signal and generates an error signal responsive to the comparison. Reference numeral 12 is a reference error signal supplier circuit which generates a reference error signal representing a predetermined admissible range (for example, ± R in FIG. 4) of the error between the detected glass ribbon width and the standard and reference numeral 13 is a comparator circuit which compares the error signal from the circuit 11 with the reference error signal from the circuit 12 and transmits the error signal, when the error signal is within the admissible range of the error, to a first control circuit 14 which generates a first control signal which is proportional to an average value of a series of the error signals or the time integral of the error signal and transmits the error signal to a second control circuit which generates a second control signal in accordance with a control mode of a proportional-plus-integral action, of a sampled-data system, when the error signal is out of the range of the error. Reference numeral 22 is a temperature difference signal generator circuit which receives a temperature signal generated once every predetermined sampling period by the radiation thermometer 20 and generates a temperature difference signal representing a difference between any of two successive temperature signals. Reference numeral 23 is a level indicator mounted in the melting furnace for measuring the level of a molten glass in the furnace and reference numeral 24 is a level difference signal generator circuit which receives a level signal generated once every predetermined sampling period by the level indicator 23 and generates a level difference signal representing a difference between any of two successive level signals. Reference numeral 16 is a tweel control circuit which receives as its input the temperature difference signal from the circuit 22, the level difference signal from the circuit 24 and the control signal from either of the first control circuit and the second control circuit and activates the driving device 5 in response to these input signals to move the tweel up and down so as to maintain the glass ribbon width uniform.

Referring to FIG. 4, reference letter O denotes the standard width of the glass ribbon and +R and -R denote respectively an upper set point and a lower set point of the admissible range of the error. Preferably the tweel 4 is regulated to move up or down in predetermined control cycles, as shown by broken lines, and the control cycle time is chosen slightly greater than the length of time from a change of the tweel height to a responsive change of the glass ribbon width, that is, a time necessary for a molten glass to travel from the tweel 4 to the location of the video-analizers. Preferably the error signal is generated once in every sampling period which is chosen much smaller than the control cycle time. In this case, the sampled data system of the second control circuit periodically picks up the error signals as sampled data.

In the thus arranged control system, when the temperature of the molten glass detected by the radiation thermometer increases or decreases, the tweel 4 is regulated to move up or down in response to the temperature difference signal from the circuit 17. Also when the level of a molten glass in the melting furnace detected by the level indicator increases or decreases, the tweel is regulated to move up or down in response to the level difference signal from the circuit 19.

Furthermore the error signal generator circuit 11 generates an error signal, for example, when the detected width of the glass ribbon 8 goes beyond the standard width because of an increase of the amount of molten glass poured from the canal 1 through the tweel 4 onto the molten metal. And if this error signal is within the admissible range (for example, O to +R in FIG. 4), then the first control circuit receives the error signal and is brought into operation to generate a first control signal in accordance with its control mode. The tweel is regulated to move down in response to this control signal from the first control circuit 11. If the error signal is over the admissible range (for example, greater than +R in FIG. 4), then the second control circuit 15 receives the error signal and is brought into operation to generate a second control signal, as shown as rectangular pulse signals in FIG. 4, in accordance with its control mode. And the tweel is moved down during a time interval represented by the pulse width of the control signal. Almost the same operational process is carried out when the tweel is moved up because of a decrease of the molten glass amount.

In the above mentioned embodiment, the video analizers 7, 7' are employed to detect positions of both side edges of the glass ribbon but this invention is not limited by this.

According to the present invention, as will be understood from the foregoing description, the tweel is regulated in response to a detected temperature of a molten glass poured to the molten metal bath and thus the present invention enables one to reduce a fluctuation of the glass ribbon width by compensating a change of the molten glass temperature before it affects the rate of molten glass supply which in turn affects the glass ribbon width.

Furthermore an admissible range of the error between the glass ribbon width and the desired standard is predetermined and to minimize a fluctuation of the glass ribbon width the control mode is changed depending on whether or not the error signal is within the predetermined range. That is, when the error signal is within the predetermined range, the tweel is regulated by the control signal which is generated by the first control circuit on the basis of the average value of the error signal or the integral of the error signal, so that the tendency toward a fluctuation of the glass ribbon width is restrained. When the error signal is outside the predetermined range, the control signal is generated by the second control circuit in accordance with a proportional-plus-integral control mode, of a sampled-data system, so that the glass ribbon width is reliably prevented from going further beyond the predetermined admissible range. Thus, according to the present invention, the fluctuation of the glass ribbon width is minimized and therefore the yield of flat glass products is enhanced. 

What is claimed is:
 1. A method for controlling the width of a glass ribbon in a float process of flat glass production in which a molten glass is poured through a tweel onto a molten metal in a bath to form a glass ribbon floating and advancing on the molten metal bath, the method comprising:detecting the width of the glass ribbon at a location remote from the tweel, comparing the detected width with a predetermined standard and generating an error signal representing the error therebetween, discriminating between a first case where said error signal is within a predetermined range of the error and a second case where said error signal is outside said range, generating a first feedback control signal from said error signal in accordance with an integral control mode in the first case and a second feedback control signal obtained from said error signal in accordance with a proportional-plus-integral control mode of a sampled data system in the second case, detecting the temperature of a molten glass to be supplied to the bath and generating a temperature signal representing the detected temperature, generating a first feedforward control signal from said temperature signal, and regulating the tweel to manipulate the rate of supplying a molten glass to the bath under command of said first feedback control signal in the first case and said second feedback control signal in the second case in such a manner as to make said error disappear and additionally under command of said first feedforward control signal in such a manner as to compensate the influence of the temperature change of the molten glass to be supplied to the bath.
 2. A method as claimed in claim 1, wherein said error signal is generated periodically once every predetermined sampling period, and wherein said error signals are further sampled periodically to generate said second feedback control signal.
 3. A method as claimed in claim 2, wherein said feedback control signal is generated periodically once every predetermined control period.
 4. A method as claimed in claim 3, wherein said sampling period is much smaller than said control period.
 5. A method as claimed in claim 4, wherein said control period is slightly greater than a time interval necessary for a molten glass to advance from the location of the tweel to the location where the width of the glass ribbon is detected.
 6. A method as claimed in claim 5, wherein said temperature signal is generated periodically once every predetermined sampling period and wherein said first feedforward control signal is proportional to a difference between any of two successive temperature signals.
 7. A method as claimed in claim 6, wherein the temperature of a molten glass is detected near the location of the tweel.
 8. A method as claimed in claim 1, further comprising:detecting the level of a molten glass in a tank which contains a molten glass and supplies it to the bath and generating a level signal representing the detected level, and generating a second feedforward control signal from said level signal, and wherein the tweel is regulated additionally under command of said second feedforward control signal in such a manner as to compensate the influence of the level change of the molten glass in the tank.
 9. A method as claimed in claim 8, wherein said level signal is generated periodically once every sampling period and wherein said second feedforward control signal is proportional to a difference between any of two successive level signals. 